The session timeout counter is automatically maintained by the application server and cannot be modified by application code.
However, if you want to post an AJAX-refreshed display element and still timeout when no useful work has been done, you can always maintain your own countdown timer in the AJAX action processor. When your timer drops to zero, manually log out the user.
You may want to install a servlet filter or phase listener to reset your counter when legitimate page requests are received so that each page doesn't require its own manual counter reset logic.
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